Scientists have long believed that the Moon was formed by a massive object crashing into the Earth billions of years ago and sending chunks hurtling into space that ultimately coalesced.

But what was that thing that hit the Earth 4.5 billion years ago — commonly nicknamed Theia? How big was it? What was it made of? What was its source? There have never been any answers there, because Theia was completely destroyed when it crashed into the Earth.

But researchers from the University of Chicago and other institutions have a better idea now, having analyzed moon rocks from the Apollo missions.

The researchers from UChicago, the Max Planck Institute for Solar System Research, and the University of Hong Kong have concluded that Theia was a small planet that most likely formed in the solar system near the Earth, but maybe a little closer to the sun.

As UChicago explained, every celestial body — whether a star, a planet, or a meteor — has its origins preserved in its chemical makeup. But getting information from that chemical makeup is not as easy as it may sound, as conclusions can't be drawn just based on figuring out what elements are there.

Scientists instead have to break down the chemical makeup into isotopes that were forged in stars long ago.

What's an isotope? Different isotopes of an element all have the same number of protons, but a different number of neutrons, and thus, a different atomic mass.

For instance, as explained by the U.S. Department of Energy, carbon always has six protons. But there are three naturally occurring isotopes of carbon — carbon-12, which has six neutrons; carbon-13, which has seven; and carbon-14, which has eight.

Isotopes differ in more ways than mass. The Department of Energy notes that carbon-12 is stable, but carbon-14 is not. Carbon-14 undergoes radioactive decay over time, and for that reason, it is used to date ancient or prehistoric objects.

In the case of celestial objects, isotopes are key because after they were ejected by stars, they were never fully mixed within the solar system, UChicago explained. This means different regions of the solar system ended up with different proportions of different isotopes of elements, and thus, the prevalence of specific isotopes can be used to determine the origins of celestial objects, UChicago explained.

To learn more about Theia, the research team analyzed terrestrial rocks from the Earth, six lunar rock samples, and samples of meteorites from different areas of the solar system where Theia may have been formed, UChicago said.

The team separated out and measured iron isotopes in the specimens.

As explained by UChicago, separating out isotopes is a big pain. Since isotopes can only differ by the weight of a few neutrons, and the samples of the Moon that have to be examined are so "tiny and precious," acute precision is required, the university said.

But the researchers stepped up and did it. They made precision measurements of iron isotope ratios in the samples, and combined them with previously measured isotope ratios of five other elements — chromium, calcium, titanium, molybdenum, and zirconium.

The researchers drew conclusions from these measurements based on their knowledge of how these different metals are involved in planetary processes, the university said. For instance, UChicago said, the iron and molybdenum found in the early earth were most likely formed in the planet's core before Theia struck, and thus, the iron found in the crust and mantle of the earth today probably came from the strike by Theia.

Further calculations determined that Theia most likely itself formed in the inner solar system, closer to the sun than the Earth itself did. How did they figure this out?

In a UChicago news release, former UChicago and current University of Hong Kong professor Nicolas Dauphas noted that for our contemporary purposes, the Theia collision worked out pretty well for everything.

"During the early solar system's game of cosmic billiards, Earth was struck by a neighbor," Dauphas said in the release. "It was a lucky shot. Without the Moon's steadying influence on our planet's tilt, the climate would have been far too chaotic for complex life to ever flourish."

After all, the Moon is more than a pretty sight in the sky in whose light we might dance in a fine and natural sight, to quote a 70s pop classic. For instance, the gravitational pull of the Moon is responsible for tides in the oceans, and the Universities Space Research Association notes that the pull of the Moon is also responsible for the length of a day — which has grown longer as the distance between the Earth and the Moon has increased.

Meanwhile, the USRA also notes that the Theia collision may not only have formed the moon, but tipped the Earth to tilt the North Pole 23.5 degrees off from straight upward, thus giving the Earth its seasons.

Indeed, we've come a long way from the days when people said the moon was made of cheese — though most likely, nobody ever actually really believed that.